Salvation at Work in You

As we have been looking at Philippians we have already seen that God calls us to unity within the body of Christ (Philippians 1:27). The only way that we achieve unity is through humility. Paul has given us the greatest example of humility by reminding us about the humiliation of Christ and it is that same attitude that believers are suppose to have (Philippians 2:1-8). Now we come to vs 12-13 and there is a call for us to "work out our salvation". This command is still in the context of unity and humility. What exactly does "work out our salvation" mean? We are all faced with a difficulty because it says for us to "work out.." but also it says that it is "God who works in us..." Vs 13 how do we reconcile both those things? How do we "work out our salvation"? How do we know if God is at work in us? What does this have to with humility and unity?